and toads in the Cascade Mountains, had observed that some long-standing popu- lations simply weren't there anymore. "The debate was whether or not there re- ally was an amphibian population prob- lem, because some people were saying it was just natural variation." At the point that Karen Lips went to look for her first research site, she purposefully tried to steer clear of the controversy. "I didn't want to work on amphibian decline," she told me. "There were endless debates about whether this was a function of randomness or a true pattern. And the last thing you want to do is get involved when you don't know what's going on." But the debate was not to be avoided. Even amphibians that had never seen a pond or a forest started dying. Blue poison-dart frogs, which are native to Su- riname, had been raised at the National Zoo, in Washington, D.C., for several generations. Then, suddenly, the zoo's tank-bred frogs were nearly wiped out. I t is difficult to say when, exactly, the current extinction event-some- times called the sixth extinction-began. What might be thought of as its opening phase appears to have started about fifty thousand years ago. At that time, Austra- lia was home to a fantastic assortment of enormous animals; these included a wom- batlike creature the size of a hippo, a land tortoise nearly as big as a VW Beetle, and the giant short-faced kangaroo, which grew to be ten feet tall. Then all of the continent's largest animals disappeared. Every species of marsupial weighing more than two hundred pounds-there were nineteen of them-vanished, as did three species of giant reptiles and a flightless bird with stumpy legs known as Genyornis newtoni. This die-off roughly coincided with the arrival of the first people on the continent, probably from Southeast Asia. Australia is a big place, and there couldn't have been very many early settlers. For a long time, the coincidence was dis- counted. Yet, thanks to recent work by geologists and paleontologists, a clear global pattern has emerged. About eleven thousand years ago, three-quarters of North Americà s largest animals-among them mastodons, mammoths, giant bea- vers, short-faced bears, and sabre-toothed tigers-began to go extinct. This is right around the time the first humans are be- - y .,- .",. -------- --....-- -----------.... ...------ ---.... -- <. -------------- ---- ---- "--- t '-- '- --:::::--- 0 \. " · '/l4'\ ' , JJ a. < . . t. . "'." ,. 1.' f '----"-- J' tç ._ I r / '\ --: = - .. - ., --E:3- '..' 2 / /' ,. . . - -::- - _ 7 _ . . . -------------- \ \ 1/ ' O r - 1\\ -- - ----... -----> ---- --- "-- - ------- ------ ----.. r --- \ 2- flfllfff '1 wish they'd quit sending myfinancial statements." . lieved to have wandered onto the conti- nent across the Bering land bridge. In rel- atively short order, the first humans settled South America as well. Subsequently, more than thirty species of South Amer- ican "megamammals," including elephant- size ground sloths and rhino-like creatures known as toxodons, died out. And what goes for Australia and the Americas also goes for many other parts of the world. Humans settled Mad- agascar around two thousand years ago; the island subsequently lost all mammals weighing more than twenty pounds, in- cluding pygmy hippos and giant lemurs. "Substantial losses have occurred through- out near time," Ross MacPhee, a curator at the American Museum of Natural His- tory, in New York, and an expert on ex- tinctions of the recent geological past, has written. "In the majority of cases, these losses occurred when, and only when, peo- ple began to expand across areas that had never before experienced their presence." The Maori arrived in New Zealand around eight hundred years ago. Theyencoun- tered eleven species of moas-huge os- trichlike creatures without wings. Within a few centuries-and possibly within a sin- . gle century-all eleven moa species were gone. While these "first contact" extinc- tions were most pronounced among large animals, they were not confined to them. Humans discovered the Hawaiian Islands around fifteen hundred years ago; soon af- terward, ninety per cent of Hawaiî s native bird species disappeared. 'We expect extinction after people ar- rive on an island," David Steadman, the curator of ornithology at the Florida Mu- seum of Natural History, has written. "Survival is the exception." Whywas first contact with humans so catastrophic? Some of the animals may have been hunted to death; thousands of moa bones have been found at Maori ar- cheological sites, and man -made artifacts have been uncovered near mammoth and mastodon remains at more than a dozen sites in North America. Hunting, how- ever, seems insufficient to account for so many losses across so many different taxa in so many parts of the globe. A few years ago, researchers analyzed hundreds of bits of emu and Genyornis newtoni eggshell, some dating from long before the first people arrived in Australia and some from after. They found that around THE NEW YORKER, MAY 25, 2009 57